1. Field of the Invention
The present invention generally relates to a light emitting device and a method for producing the same. More specifically, the invention relates to a light emitting device wherein a light emitting element is combined with a phosphor (or a fluorescent substance) which is designed to receive light from the light emitting element to emit light having a different wavelength from that of the light emitted from the light emitting element, and a method for producing the same.
2. Description of the Prior Art
In recent years, there has been actively developed a light emitting device for emitting white light by combining a light emitting element, such as a light emitting diode (LED) for emitting light in wavelength ranges from near ultraviolet to blue, with a phosphor (or a fluorescent substance) which is designed to receive light from the light emitting element to emit light having a different wavelength from that of the light emitted from the light emitting element. In such a white light emitting device, a LED chip is mounted on the bottom face of a cup-shaped recessed portion formed in a lead frame or substrate, and a resin mixed with a phosphor, which is designed to absorb light from the LED chip to emit light having a longer wavelength than that of the light from the LED chip, is potted in the cup-shaped recessed portion to seal the LED chip therein.
If such a white light emitting device is used as a light source for illumination, it is desired that the device can emit light having a high emission power (a high illumination intensity or illuminance), a small irregularity in color, and good color rendering properties. In order to cause light from such a white light emitting device to be white light having good color rendering properties, it is proposed that a resin mixed with two kinds of phosphors for emitting light having different wavelengths by light from a LED chip is arranged around the LED chip (see, e.g., Japanese Patent Laid-Open Nos. 2000-244021, 2001-127346, 2003-101081, 2003-318447 and 2004-152993).
It is also proposed that a transparent or translucent resin containing large phosphor particles and small phosphor particles for absorbing at least part of light from a LED chip to emit light having different wavelengths is arranged around the LED chip, so that the large phosphor particles are distributed near the LED chip to form a color conversion layer to efficiently carry out color conversion while the small phosphor particles are distributed outside of the color conversion layer to suppress irregular color (see, e.g., WO02/059982).
Moreover, there is proposed a light emitting device for emitting white light by arranging a resin mixed with a YAG phosphor (a rare-earth aluminate phosphor activated by a rare earth element having a garnet structure), which is designed to emit light in wavelength ranges from green to yellow as a complementary color light with respect to light in the blue range, around a light emitting element which is designed to emit light in the blue range. There is also proposed a white light emitting device capable of realizing a luminescent color having higher color rendering properties by mixing a resin with a phosphor of Ca2Si5N8:Eu or the like which is designed to emit light in wavelength ranges from orange to red (see, e.g., WO2005/029596).
In order to improve the luminance of a white light emitting device, large phosphor particles having a high light-transforming efficiency are preferably used as the phosphor particles. However, if a resin mixed with large phosphor particles is cured, the phosphor particles are easily precipitated in the resin while the resin is cured, so that the variation in amount of the phosphor particles is caused in the resin. For that reason, the quantity of light, the wavelength of which is converted by the phosphor particles, is changed every optical path through which light emitted from the LED passes, so that irregular color is caused.
If a resin having a high viscosity is used in order to prevent the precipitation of the phosphor particles, bubbles are mixed in the resin, so that light entering the bubbles is confined therein to deteriorate the emission power of the light emitting device. If a resin having a high viscosity is applied by means of a dispenser or the like, it is difficult to discharge the resin from a nozzle, so that it is difficult to control the amount of the resin to be applied. Therefore, there is a problem in that it is difficult to control the color temperature of the light emitting device.
On the other hand, if small phosphor particles are used as the phosphor particles, it is possible to prevent irregular color due to the precipitation of the phosphor particles in the resin. However, if the particle diameter of the phosphor particles decreases, the light-transforming efficiency generally deteriorates, so that there is a problem in that the emission power of the produced white light emitting device is deteriorated.
In addition, in a white light emitting device wherein a light emitting element is mounted on the bottom face of a cup-shaped recessed portion, which is formed in a lead frame or substrate, to be sealed with a resin containing phosphor particles by potting the resin, it is required that the optical path length, by which light emitted from the light emitting element passes through the resin, is constant in order to reduce the irregularity in color. In order to make it constant, it is required that the top face of a sealing member of a resin is raised to be convex so that the distance between the light emitting element and the surface of the sealing member is constant.
However, when the light emitting element arranged in the cup-shaped recessed portion is sealed with a resin by potting, it is not easy to control the shape of a sealing member of the resin so that the optical path length, by which light emitted from the light emitting element passes through the resin, is constant, due to the viscosity of the used resin and the gravity applied to the resin. As a result, the irregular color of light from the light emitting device is caused.